The far south-west of Britain, comprising the modern counties of Cornwall and Devon, and at one point Somerset, formed a separate kingdom known as Dumnonia in this period. Dumnonia was the home of a confederation of tribes called the Dumnonii (which has been translated as "the dominators"), but its history is very poorly chronicled – no historical accounts originating from the kingdom itself survive, so our knowledge of it is based on glancing mentions in other sources, most notably a handful of charters and a couple of entries in the Anglo-Saxon Chronicle, together with the evidence of surviving inscriptions and of archaeology. One consequence is that we have no exact understanding of when the kingdom came into existence or even exactly when it was subsumed into the lands controlled by Wessex. The most specialists will usually concede is that Dumnonia was around well before 550 and that it survived in some form (after c.700 comprising lands only as far north as the river Tamar, roughly the boundaries of modern Cornwall) until as late as the first half of the ninth century. At some point prior to this the Cornish kings almost certainly became subordinate to the West Saxon monarchs.

The early kingdom had close connections with other Celtic polities, including those in south Wales and Brittany. There is considerable duplication in place names between sites in Dumnonia and sites in Brittany - in this period the place names of the latter included "Cornouaille" and "Dumnonia" itself. But there was apparently also a lingering memory of the Roman occupation; Dumnonia's first known king had a Roman name – Constantine.

I wrote about the early history of Dumnonia here, taking the story up to about 600. Wendy Davies covers the outline of next century in the New Cambridge Medieval History in just a couple of lines: "In 658 the West Saxon king Cenwalh was fighting on the River Parrett in Somerset; by 690 there was an English monastery in Exeter; and in 710 the West Saxon king Ine was making land grants near the River Tamar, the traditional eastern boundary of Cornwall."

So early Dumnonia was a Celtic kingdom, and a surprisingly large one by the standards of the time, stretching for more than 125 miles from end to end. With regard to the kingdom's later history, in the 700s and 800s, Frank Stenton stresses the distinction between Devon and Cornwall, based on place-name evidence, which he suggests shows the comprehensive Anglicisation of the former relative to the latter during the eight and ninth centuries. He suggests this was a product of a significant migration of Saxons into Devon after 700: "Although Celtic place-names survived the conquest in considerable numbers, they remain as exceptions, distributed over the county in a way which shows that English settlement was equally thorough in its eastern and western portions." [Note that a more modern interpretation of this period – admirably laid out by /u/alfriclofgar in a long post here – would see the hostile forces as the product of a long process of migration, intermarriage and state-forming among an essentially Saxon-British population; I use "Saxon" as a term of convenience, but this was not some sort of "race war"...] A Saxon bishopric, anyway, was established at Sherbourne in 705, and in 710 the kings of Wessex and Sussex combined forces to attack the king of what was left of Dumnonia. But these wars were not one-way affairs. The Welsh Annales Cambriæ records a British victory that took place at a place called Hehil in 722.

In the period you are writing about the key events revolve around the Saxon king Ecgberht - Egbert (d.839) - a unifier who annexed Kent and harried Cornwall in 814-15. This was not enough to subjugate Dumnonia or deprive its kings of their freedom of action, however; there are records of British raids into Wessex in 825, and in 839 the men of Dunmonia allied themselves with the Danes against the Saxons and invaded Wessex. The Dumnonian army was defeated by Egbert at Hingston Down, on the Tamar, in that year; Stenton says that "it was probably this victory which made Cornwall finally part of England, for there is no evidence of any later movements for Cornish independence, and 50 years after the Battle of Hingston Down King Alfred's will deals as freely with lands in Cornwall as with any of the ancient possessions of his house."

Malcolm Todd, in The South-West to 1000, broadly agrees: "We still hear of Cornish kings, such as Dumgarth, in the later ninth century, but they can only have been dependents of the Kings of Wessex, reigning as vassals in a conquered land." And he adds an important comment on the nature of the struggle: "The long drawn out contest between Wessex and Dumnonia, as it is sketched for us in the Chronicle, is not the unfolding of a determined and expansionist Westpolitik carried out by the military might of English kings at the expense of their weaker neighbours. More fundamentally, it was a struggle for the possession of land by rival owners, some British, some English, all liable to resort to force." The History Files site has a really nice – albeit heavily conjectural – history of "Dumnonia in Maps" that may help you visualise all this, though the idea of clearly defined and fixed boundaries that can be mapped in this way isn't really applicable to this period.

For your scene of 821, anyway, it seems safe to say that you should describe Tintagel as in Dumnonia, not Wessex and definitely not in "England," which didn't really exist till a century later. We have no clear idea of who the Dumnonian ruler was in this period, unfortunately; the best history can offer is the name Hopkin ap Hernam, who supposedly ruled around 810, but whose name first appears on a king list dating to no earlier than the 17th century. If you wanted to sketch in any background, it would probably be not unrealistic to paint a scene with a fin-de-siècle feel, and perhaps incorporate some portents of tough times to come.